i installed this (https://github.com/alex47/KDE-Rounded-Corners) effect but it doesn't work ( I get black corners, and my composition manager is running). Also there are no instructions on how to remove it.
I already went in Desktop Effects and disabled "ShapeCorners"( the effect i installed was a fork of it) but still nothing changes.

You may be able to run "sudo make uninstall" to uninstall it. If that doesn't work, try running "sudo make install", which should print out what files it's intalling, then manually remove the files listed.

(side note: it's a shame that kde projects don't look in /usr/local for plugins such as these as that significantly reduces the danger when installing or removing 3rd-party plugins from source)